A calcined gypsum is manufactured by separately calcining or mixing and calcining raw gypsums such as chemical gypsums and natural gypsums so that the raw gypsum which is mainly dihydrate gypsum is principally converted into hemihydrate gypsum.
An appropriate amount of water is added into the calcined gypsum obtained as described above to provide a slurry which is a dihydrate and hardens rapidly and therefore is preferably used for raw materials of various gypsum products, for example, used as a raw material for manufacturing of a gypsum board.
A gypsum board is a board-shaped one in which a core based on dihydrate gypsum is covered with a base paper for gypsum board, wherein, as for manufacturing thereof, a calcined gypsum, a bonding aid, a hardening accelerator, foam for lightening thereof, another additive, and the like, and further an admixture and water are kneaded, and thus obtained calcined gypsum slurry is poured into between upper and lower base papers for gypsum board, shaped into a board-like one, and subsequently cut roughly after hardening thereof, and cut into a product dimension after forced drying thereof.
The gypsum boards manufactured as described above have been widely used as interior materials for building in view of the fire-protection and fire-proof properties, sound-insulating property, workability, economical efficiency thereof and the like. Recently, while the recycle rate of waste gypsum boards at the time of new construction or demolition to gypsum board manufacturers has increased due to the social demand of volume reduction of industrial wastes, the compounding ratio of recycled gypsum in a raw gypsum for gypsum board is increased accordingly. However, with increase of the recycle rate, there are problems of increase of the amount of mixing water for calcined gypsum to provide a calcined gypsum slurry with an appropriate fluidity, mainly caused by a fine gypsum crystal of a recycled gypsum, increase of drying energy required for increasing extra water that has to be dried out in a process for drying a gypsum board, and considerable lowering of the productivity of a gypsum board.
The following prior art is provided for reducing the amount of mixing water for calcined gypsum which is used for manufacturing a gypsum board.
For example, Japanese Examined Patent Application No. H03-51665 discloses a method for manufacturing a calcined gypsum characterized by including a process for conveying raw materials of calcined gypsum to a blender, a process for mixing water into the raw materials of calcined gypsum in the range of about 1% to about 10% by weight of the raw materials and conducting heal treatment of the surfaces of calcined gypsum particles with a small amount of free water for a short time period to prevent the particles from being miniaturized needlessly and dissolved in water, a process for drying the calcined gypsum which has been subjected to the heal treatment at an elevated temperature, and a process for crushing the dried and heal-treated calcined gypsum to increase the surface areas of gypsum particles whereby the surfaces increase the rate of increase of the inner strength of a gypsum board and reacting it with water to increase the final strength thereof whereby the amount of mixing water can be reduced by about 20% compared to a calcined gypsum that is not treated by means of water addition. However, in this method, there is a problem that the rate of manufacturing of a gypsum board is forced to be reduced by increase of the setting time although the mixing water amount of a calcined gypsum can be reduced and a necessary drying energy can be reduced, or a large quantity of a setting accelerator has to be used in order not to reduce the rate of manufacturing of a gypsum board, thereby degrading the various physical properties of the gypsum board. Furthermore, a new water addition treatment apparatus and a dryer for drying out free water remaining in a calcined gypsum after the water addition treatment are needed for this method, and there is a problem that the complexity of equipment or increase of the installation cost thereof is caused thereby increasing the cost for manufacturing a calcined gypsum.